The studies of the absorption and secretion rates of calcium and inorganic phosphate in the human small intestines by means of intestinal perfusion will be continued. Particular attention will be devoted to the hormonal effects on these absorption and secretion rates. The effects of the active metabolites and analogs of vitamin D3, 25-hydroxyvitamin D3 (25 OHD3), 1,25 dihydroxyvitamin D3 (1,25-(OH)2D3) and 1-alpha, hydroxyvitamin D3 (1 alpha OHD3), on the absorption rates of calcium and inorganic phosphate will be studied in normal subjects and patients with disorders of mineral metabolism. Another series of perfusion will focus on the intestinal absorption of inorganic phosphate in order to increase the knowledge about phosphate absorption in the human small intestines. Experimental factors which may increase or inhibit phosphate absorption, such as the intraluminal concentrations of other ions, especially calcium or magnesium, the intraluminal pH and osmolality and the presence or absence of dietary nutrient, will be studied in detail. The systematic perfusion studies of phosphate absorption will be conducted in normal volunteers. Patients with alterations in vitamin D metabolism, e.g., hypoparathyroidism, will be studied before and after the administration of one of the active forms of vitamin D. The rates of intestinal absorption or secretion of calcium and inorganic phosphate will be determined in the aging normal subjects and age and sex matched subjects with osteoporosis. The response of the observed rates in these latter subjects to doses of vitamin D will also be evaluated. The concomitant measurement of the luminal concentration of the cyclic nucleotides and the calculated absorption rates of water and ions should provide correlations which may facilitate the recognition of the mechanisms underlying the normal and abnormal absorptive processes. The proposed research will clarify and expand our knowlege of calcium and inorganic phosphate absorption in the human small intestines in health and selected disease states.